1. Field of Application
The present invention relates to an AC generator, generally referred to as an alternator, of a vehicle such as an automobile or truck. In particular, the invention relates to the brush apparatus of such an alternator.
2. Description of Related Art
The brush apparatus of the alternator of a vehicle incorporates a pair of brushes held in sliding contact and in electrical contact with respective slip rings that are mounted on the rotor of the generator. Current flows from one of these brushes (referred to in the following as the positive polarity brush), from a high (positive) power supply potential, into the corresponding slip ring, and the current flows from the other brush (referred to in the following as the negative polarity brush) into the corresponding slip ring, and hence to the low power supply potential. The slip rings corresponding to the positive polarity brush and the negative polarity brush will be respectively referred to as the positive polarity slip ring and negative polarity slip ring.
Hitherto, such a pair of brushes have been generally formed of identical materials and with identical configurations, for example as described in Japanese patent publication No. 9-182353 (pages 4-8), FIGS. 1-8).
Over a long period of use, wear occurs at the area of contact between such a brush and the corresponding slip ring. The operating life of a brush (more accurately, the operating life of the combination of that brush with the corresponding slip ring) until the brush becomes defective as a result of such contact wear, will be referred to in the following as the wear-down lifetime of the brush. Due to the fact electrical current flows in respectively opposite directions through the pair of brushes, the negative polarity brush and positive polarity brush have respectively different wear-down lifetimes. Thus there is the problem that when one of the brushes (more specifically, the combination of a brush and the corresponding slip ring) reaches the end of its wear-down lifetime, the alternator as a whole becomes inoperable. Hence the operating life of the alternator is shortened.
In Japanese patent publication No. 2005-261153, it is proposed to overcome this problem by forming the slip rings from respectively different materials, with the slip ring corresponding to the brush having the more rapid rate of wear being formed of a metallic material that has greater resistance to wear than the material of the other slip ring. However such a method has basic disadvantages.
Firstly, if the metallic material having greater resistance to wear is for example stainless steel, then the material has a substantially higher electrical resistivity than a material such as copper. This will result in an amount of lowering of the output power of the alternator, to an extent that cannot be ignored.
Secondly, with the usual method of manufacture of the slip rings for an alternator, a single elongated ring is formed in a mold, then a central part of the ring is removed by machining, to form the negative polarity-side slip ring and positive polarity-side slip ring. However if the slip rings are formed of respectively different materials, then it becomes necessary to provide separate molds for the pair of slip rings, and to form these in the molds to a high degree of accuracy of axial concentricity. Hence, the molding process becomes slower and more complex, and the manufacturing cost will thereby be increased.
Thirdly, it is necessary to electrically connect the slip rings to the field winding of the generator by welding or soldering attachment of connecting leads to the slip rings. However if the negative polarity-side slip ring and positive polarity-side slip ring are of respectively different metallic materials, then it becomes necessary to provide two separate installations for performing such attachment operations, since the requirements for performing the welding or soldering work will be respectively different for the two types of metallic material. Thus since the work cannot be performed by using a single installation for performing the welding or soldering attachment operations, the manufacturing cost will be increased.
Fourthly, if the negative polarity-side slip ring and positive polarity-side slip ring are of respectively different metallic materials, then it becomes necessary to provide two separate installations for performing machining work to form the slip rings, since the machining requirements will be respectively different for the two types of metallic material. Hence, the manufacturing cost will be increased.